This invention relates generally to a mixer for mixing granular material into a liquid. More particularly, the invention relates to a mixer adapted to mix powdered or ground rubber with hot, liquid asphalt.
Mixers of this general type are useful in producing a rubber-asphalt mixture which is suitable for use in, for example, road paving projects. Such a mixture can have improved strength and resilience characteristics over asphalt alone. In addition, use of such a mixture aids in disposing of used tire rubber. However, use of rubber-asphalt mixtures have been limited, in part, due to difficulties in thoroughly mixing the ground rubber with the liquid asphalt. The rubber which is mixed with liquid asphalt is a very fine powder and is typically ground to a size which will pass through a 40 to 80 mesh screen. As a result, the ground rubber has a relatively large surface area which must be thoroughly "wetted" by the liquid asphalt to create a homogenous mixture. Clumps or balls of rubber form if the surface area of the rubber is not sufficiently wetted.
In general, prior rubber-asphalt mixers draw liquid asphalt from a cargo or holding tank into a separate heated tank. The ground rubber is typically introduced into the separate tank by simply dropping the rubber on top of the liquid asphalt. Because the rubber is relatively light, it floats on top of the asphalt until it is drawn downwardly into the asphalt by a mixing device.
One such prior mixer utilizes a relatively large tank (e.g., eight to ten feet in diameter) and relatively large rotating paddles (e.g., two to three feet in length) to mix ground rubber with liquid asphalt. The paddles create a downwardly spiraling whirlpool which draws the ground rubber into the asphalt. Substantial energy is wasted in this mixer because the relatively large paddles tend to cause the entire body of liquid asphalt to revolve in the tank. Moreover, substantial energy is expended in this mixer by virtue of the relatively large paddles rotating in the mixture at a constant speed as the viscosity of the mixture increases due to an increasing percentage of rubber in the mixture.
Smithers et al U.S. Pat. No. 4,506,982 discloses a mixer which utilizes a relatively small, high-speed blending device to draw the ground rubber downwardly into the liquid asphalt and to mix the rubber and the asphalt together. While the Smithers et al arrangement provides for relatively efficient mixing of the rubber and the asphalt, such an arrangement requires the use of a separate heated tank, a cart to carry the tank, and a substantial number of pumping and associated flow control components to draw the asphalt into the tank and to pump the rubber-asphalt mixture out of the tank.